For apps that have significant offline requirements, this functionality can save a significant amount of time and significantly reduce the amount of code that needs to be written and more importantly maintained.
One of my clients just recently seriously considered going with Firebase. The choice came down to Firebase and Couchbase, which has an even better offline functionality. After reading this post I am glad that we didn't going with Firebase.
Of course Couchbase has their own problems. The supported version of Couchbase is prohibitively expensive for small organizations and Couchbase themselves strongly recommended against deploying the community edition. (The community edition also artificially lags behind the supported edition 6 months. Couchbase even delays critical security fixes that same 6 months.)